This invention relates to a wheel assembly and, more particularly, relates to a synthetic resin material molded wheel construction having a static electricity discharge feature associated therewith.
Wheel constructions for a wheel primarily used in casters produced under high production conditions are, in many cases, used in association with shopping carts and other material handling carts. In instances where the high production wheels are made of a synthetic resin material, both the wheel and the floor on which it is rolling are generally non-conductive and, as a result, when non-conductive materials come into contact and separate, they will cause a division in protons (positive charge) and electrons (negative charge) in the materials as they come into contact and separate. This process is known as triboelectric generation. In other words, as the shopping cart is moved across the floor, the wheels and the floor are non-conductive and the wheel and the flooring trade electrical charges with the floor becoming one polarity and the cart assuming the opposite polarity. During this process, the charge build up in the wheels will transfer to the tubular steel frame of the cart to the plastic or metal cage of the cart. The plastic or metal cage has the ability to act as a capacitor and store a significant electrical charge. Since the wheels isolate the cart cage from ground, the cart cage can accumulate a significant surface charge because it is not readily discharged through any grounding.
Patrons using the grocery carts generally are capable of feeling static discharges when the voltage level is in excess of 3,000 volts. During testing of cart wheels on waxed concrete and tile flooring, discharges in excess of 18,000 volts have been measured.
Accordingly, it is desirable to provide a synthetic resin material high production wheel construction which has a static electricity discharge feature embedded therein and without introducing excessive cost to the wheel manufacturing process.
It is a further object of this invention to provide a wheel construction, as aforesaid, wherein the static electricity discharge feature is incorporated into the standard molding process for the synthetic resin material molded wheel construction.
It is a further object of this invention to provide a wheel construction, as aforesaid, wherein the bearing construction for the wheel is entirely electrically conductive and the static electricity discharge feature is attached to the bearing assembly and extends radially through the material of the hub and tread to engage the floor surface to effect a discharge through the aforesaid static electricity discharge feature.
It is a further object of this invention to provide a wheel construction, as aforesaid, that utilizes a static electricity discharge feature without diminishing the performance characteristic of the wheel assembly.
The objects and purposes of the invention are met by providing a wheel assembly which includes a bearing assembly for the wheel assembly housed in an electrically conductive outer shell. A hollow sleeve of synthetic resin material impregnated with electrically conductive material is operatively electrically coupled to a peripheral surface of the electrically conductive outer shell. The hollow sleeve has at least one radially outwardly extending stem integral therewith. A synthetic resin material hub and tread is configured to envelop the metallic outer shell, the hollow sleeve and the stem. A distal end of the stem terminates either flush with or projects radially outwardly of a peripheral surface of the tread to contact a surface supporting the wheel construction with each revolution of the wheel construction and to effectively facilitate a discharge of static electricity through the hollow sleeve and the stem in response thereto.